1// Copyright 2015 The Go Authors. All rights reserved. 2// Use of this source code is governed by a BSD-style 3// license that can be found in the LICENSE file. 4 5// Package rate provides a rate limiter. 6package rate 7 8import ( 9 "context" 10 "fmt" 11 "math" 12 "sync" 13 "time" 14) 15 16// Limit defines the maximum frequency of some events. 17// Limit is represented as number of events per second. 18// A zero Limit allows no events. 19type Limit float64 20 21// Inf is the infinite rate limit; it allows all events (even if burst is zero). 22const Inf = Limit(math.MaxFloat64) 23 24// Every converts a minimum time interval between events to a Limit. 25func Every(interval time.Duration) Limit { 26 if interval <= 0 { 27 return Inf 28 } 29 return 1 / Limit(interval.Seconds()) 30} 31 32// A Limiter controls how frequently events are allowed to happen. 33// It implements a "token bucket" of size b, initially full and refilled 34// at rate r tokens per second. 35// Informally, in any large enough time interval, the Limiter limits the 36// rate to r tokens per second, with a maximum burst size of b events. 37// As a special case, if r == Inf (the infinite rate), b is ignored. 38// See https://en.wikipedia.org/wiki/Token_bucket for more about token buckets. 39// 40// The zero value is a valid Limiter, but it will reject all events. 41// Use NewLimiter to create non-zero Limiters. 42// 43// Limiter has three main methods, Allow, Reserve, and Wait. 44// Most callers should use Wait. 45// 46// Each of the three methods consumes a single token. 47// They differ in their behavior when no token is available. 48// If no token is available, Allow returns false. 49// If no token is available, Reserve returns a reservation for a future token 50// and the amount of time the caller must wait before using it. 51// If no token is available, Wait blocks until one can be obtained 52// or its associated context.Context is canceled. 53// 54// The methods AllowN, ReserveN, and WaitN consume n tokens. 55type Limiter struct { 56 limit Limit 57 burst int 58 59 mu sync.Mutex 60 tokens float64 61 // last is the last time the limiter's tokens field was updated 62 last time.Time 63 // lastEvent is the latest time of a rate-limited event (past or future) 64 lastEvent time.Time 65} 66 67// Limit returns the maximum overall event rate. 68func (lim *Limiter) Limit() Limit { 69 lim.mu.Lock() 70 defer lim.mu.Unlock() 71 return lim.limit 72} 73 74// Burst returns the maximum burst size. Burst is the maximum number of tokens 75// that can be consumed in a single call to Allow, Reserve, or Wait, so higher 76// Burst values allow more events to happen at once. 77// A zero Burst allows no events, unless limit == Inf. 78func (lim *Limiter) Burst() int { 79 return lim.burst 80} 81 82// NewLimiter returns a new Limiter that allows events up to rate r and permits 83// bursts of at most b tokens. 84func NewLimiter(r Limit, b int) *Limiter { 85 return &Limiter{ 86 limit: r, 87 burst: b, 88 } 89} 90 91// Allow is shorthand for AllowN(time.Now(), 1). 92func (lim *Limiter) Allow() bool { 93 return lim.AllowN(time.Now(), 1) 94} 95 96// AllowN reports whether n events may happen at time now. 97// Use this method if you intend to drop / skip events that exceed the rate limit. 98// Otherwise use Reserve or Wait. 99func (lim *Limiter) AllowN(now time.Time, n int) bool { 100 return lim.reserveN(now, n, 0).ok 101} 102 103// A Reservation holds information about events that are permitted by a Limiter to happen after a delay. 104// A Reservation may be canceled, which may enable the Limiter to permit additional events. 105type Reservation struct { 106 ok bool 107 lim *Limiter 108 tokens int 109 timeToAct time.Time 110 // This is the Limit at reservation time, it can change later. 111 limit Limit 112} 113 114// OK returns whether the limiter can provide the requested number of tokens 115// within the maximum wait time. If OK is false, Delay returns InfDuration, and 116// Cancel does nothing. 117func (r *Reservation) OK() bool { 118 return r.ok 119} 120 121// Delay is shorthand for DelayFrom(time.Now()). 122func (r *Reservation) Delay() time.Duration { 123 return r.DelayFrom(time.Now()) 124} 125 126// InfDuration is the duration returned by Delay when a Reservation is not OK. 127const InfDuration = time.Duration(1<<63 - 1) 128 129// DelayFrom returns the duration for which the reservation holder must wait 130// before taking the reserved action. Zero duration means act immediately. 131// InfDuration means the limiter cannot grant the tokens requested in this 132// Reservation within the maximum wait time. 133func (r *Reservation) DelayFrom(now time.Time) time.Duration { 134 if !r.ok { 135 return InfDuration 136 } 137 delay := r.timeToAct.Sub(now) 138 if delay < 0 { 139 return 0 140 } 141 return delay 142} 143 144// Cancel is shorthand for CancelAt(time.Now()). 145func (r *Reservation) Cancel() { 146 r.CancelAt(time.Now()) 147 return 148} 149 150// CancelAt indicates that the reservation holder will not perform the reserved action 151// and reverses the effects of this Reservation on the rate limit as much as possible, 152// considering that other reservations may have already been made. 153func (r *Reservation) CancelAt(now time.Time) { 154 if !r.ok { 155 return 156 } 157 158 r.lim.mu.Lock() 159 defer r.lim.mu.Unlock() 160 161 if r.lim.limit == Inf || r.tokens == 0 || r.timeToAct.Before(now) { 162 return 163 } 164 165 // calculate tokens to restore 166 // The duration between lim.lastEvent and r.timeToAct tells us how many tokens were reserved 167 // after r was obtained. These tokens should not be restored. 168 restoreTokens := float64(r.tokens) - r.limit.tokensFromDuration(r.lim.lastEvent.Sub(r.timeToAct)) 169 if restoreTokens <= 0 { 170 return 171 } 172 // advance time to now 173 now, _, tokens := r.lim.advance(now) 174 // calculate new number of tokens 175 tokens += restoreTokens 176 if burst := float64(r.lim.burst); tokens > burst { 177 tokens = burst 178 } 179 // update state 180 r.lim.last = now 181 r.lim.tokens = tokens 182 if r.timeToAct == r.lim.lastEvent { 183 prevEvent := r.timeToAct.Add(r.limit.durationFromTokens(float64(-r.tokens))) 184 if !prevEvent.Before(now) { 185 r.lim.lastEvent = prevEvent 186 } 187 } 188 189 return 190} 191 192// Reserve is shorthand for ReserveN(time.Now(), 1). 193func (lim *Limiter) Reserve() *Reservation { 194 return lim.ReserveN(time.Now(), 1) 195} 196 197// ReserveN returns a Reservation that indicates how long the caller must wait before n events happen. 198// The Limiter takes this Reservation into account when allowing future events. 199// ReserveN returns false if n exceeds the Limiter's burst size. 200// Usage example: 201// r := lim.ReserveN(time.Now(), 1) 202// if !r.OK() { 203// // Not allowed to act! Did you remember to set lim.burst to be > 0 ? 204// return 205// } 206// time.Sleep(r.Delay()) 207// Act() 208// Use this method if you wish to wait and slow down in accordance with the rate limit without dropping events. 209// If you need to respect a deadline or cancel the delay, use Wait instead. 210// To drop or skip events exceeding rate limit, use Allow instead. 211func (lim *Limiter) ReserveN(now time.Time, n int) *Reservation { 212 r := lim.reserveN(now, n, InfDuration) 213 return &r 214} 215 216// Wait is shorthand for WaitN(ctx, 1). 217func (lim *Limiter) Wait(ctx context.Context) (err error) { 218 return lim.WaitN(ctx, 1) 219} 220 221// WaitN blocks until lim permits n events to happen. 222// It returns an error if n exceeds the Limiter's burst size, the Context is 223// canceled, or the expected wait time exceeds the Context's Deadline. 224// The burst limit is ignored if the rate limit is Inf. 225func (lim *Limiter) WaitN(ctx context.Context, n int) (err error) { 226 if n > lim.burst && lim.limit != Inf { 227 return fmt.Errorf("rate: Wait(n=%d) exceeds limiter's burst %d", n, lim.burst) 228 } 229 // Check if ctx is already cancelled 230 select { 231 case <-ctx.Done(): 232 return ctx.Err() 233 default: 234 } 235 // Determine wait limit 236 now := time.Now() 237 waitLimit := InfDuration 238 if deadline, ok := ctx.Deadline(); ok { 239 waitLimit = deadline.Sub(now) 240 } 241 // Reserve 242 r := lim.reserveN(now, n, waitLimit) 243 if !r.ok { 244 return fmt.Errorf("rate: Wait(n=%d) would exceed context deadline", n) 245 } 246 // Wait if necessary 247 delay := r.DelayFrom(now) 248 if delay == 0 { 249 return nil 250 } 251 t := time.NewTimer(delay) 252 defer t.Stop() 253 select { 254 case <-t.C: 255 // We can proceed. 256 return nil 257 case <-ctx.Done(): 258 // Context was canceled before we could proceed. Cancel the 259 // reservation, which may permit other events to proceed sooner. 260 r.Cancel() 261 return ctx.Err() 262 } 263} 264 265// SetLimit is shorthand for SetLimitAt(time.Now(), newLimit). 266func (lim *Limiter) SetLimit(newLimit Limit) { 267 lim.SetLimitAt(time.Now(), newLimit) 268} 269 270// SetLimitAt sets a new Limit for the limiter. The new Limit, and Burst, may be violated 271// or underutilized by those which reserved (using Reserve or Wait) but did not yet act 272// before SetLimitAt was called. 273func (lim *Limiter) SetLimitAt(now time.Time, newLimit Limit) { 274 lim.mu.Lock() 275 defer lim.mu.Unlock() 276 277 now, _, tokens := lim.advance(now) 278 279 lim.last = now 280 lim.tokens = tokens 281 lim.limit = newLimit 282} 283 284// reserveN is a helper method for AllowN, ReserveN, and WaitN. 285// maxFutureReserve specifies the maximum reservation wait duration allowed. 286// reserveN returns Reservation, not *Reservation, to avoid allocation in AllowN and WaitN. 287func (lim *Limiter) reserveN(now time.Time, n int, maxFutureReserve time.Duration) Reservation { 288 lim.mu.Lock() 289 290 if lim.limit == Inf { 291 lim.mu.Unlock() 292 return Reservation{ 293 ok: true, 294 lim: lim, 295 tokens: n, 296 timeToAct: now, 297 } 298 } 299 300 now, last, tokens := lim.advance(now) 301 302 // Calculate the remaining number of tokens resulting from the request. 303 tokens -= float64(n) 304 305 // Calculate the wait duration 306 var waitDuration time.Duration 307 if tokens < 0 { 308 waitDuration = lim.limit.durationFromTokens(-tokens) 309 } 310 311 // Decide result 312 ok := n <= lim.burst && waitDuration <= maxFutureReserve 313 314 // Prepare reservation 315 r := Reservation{ 316 ok: ok, 317 lim: lim, 318 limit: lim.limit, 319 } 320 if ok { 321 r.tokens = n 322 r.timeToAct = now.Add(waitDuration) 323 } 324 325 // Update state 326 if ok { 327 lim.last = now 328 lim.tokens = tokens 329 lim.lastEvent = r.timeToAct 330 } else { 331 lim.last = last 332 } 333 334 lim.mu.Unlock() 335 return r 336} 337 338// advance calculates and returns an updated state for lim resulting from the passage of time. 339// lim is not changed. 340func (lim *Limiter) advance(now time.Time) (newNow time.Time, newLast time.Time, newTokens float64) { 341 last := lim.last 342 if now.Before(last) { 343 last = now 344 } 345 346 // Avoid making delta overflow below when last is very old. 347 maxElapsed := lim.limit.durationFromTokens(float64(lim.burst) - lim.tokens) 348 elapsed := now.Sub(last) 349 if elapsed > maxElapsed { 350 elapsed = maxElapsed 351 } 352 353 // Calculate the new number of tokens, due to time that passed. 354 delta := lim.limit.tokensFromDuration(elapsed) 355 tokens := lim.tokens + delta 356 if burst := float64(lim.burst); tokens > burst { 357 tokens = burst 358 } 359 360 return now, last, tokens 361} 362 363// durationFromTokens is a unit conversion function from the number of tokens to the duration 364// of time it takes to accumulate them at a rate of limit tokens per second. 365func (limit Limit) durationFromTokens(tokens float64) time.Duration { 366 seconds := tokens / float64(limit) 367 return time.Nanosecond * time.Duration(1e9*seconds) 368} 369 370// tokensFromDuration is a unit conversion function from a time duration to the number of tokens 371// which could be accumulated during that duration at a rate of limit tokens per second. 372func (limit Limit) tokensFromDuration(d time.Duration) float64 { 373 return d.Seconds() * float64(limit) 374} 375